RESULTS AND DISCUSSION
3. RESULTS
FIGURE 4 | Percentages of particles from a given transect (rows) that reach a given coastal segment (columns) for backwards (A)and forwards(B) trajectories computed from the May 2013 debris observations.(C)Coastline segments that served as both sources and sinks are plotted in dark blue. Light blue segments acted as sources only and orange segments acted as sinks. Transect locations and numbers are shown in red.(D)Stick plot of winds during the particle tracking experiment. Red (blue) indicates Sirocco (Mistral) winds and the vertical dashed lines denote the time period of the AMD observations.
the majority of debris sighted inSuaria and Aliani (2014)were within 100 m of the observer. In other words, while most of the AMD was observed along either a 10 m or 30 m strip width, the abundance estimates are assumed to be representative of a 100 m strip on either side of the vessel. Particles are distributed randomly in each polygon and released at the mid-point of the transect. A sensitivity test (not shown) using particles distributed uniformly along the transect line showed no differences. In total, 9,398 particles were released, with the number of particles per transect varying from 20 to 1693.
2.5. Analysis Methods
Potential near-shore sources and sinks are identified by dividing the Adriatic coastline into 50 km segments (Figure 1B). The model domain extends to the 5 m isobath, thereby preventing identification of actual terrestrial sources and sinks. Islands large enough to be resolved by AdriaROMS model are also included, resulting in 64 coastal segments. Small islands with short coastlines, like Tremiti and Palagruza, are not included in the analysis. The open boundary at the Otranto Strait is treated as a single segment, for a total of 65 segments. For clarity, only segments that served potential sources or sinks are shown.
Percentages of particles beached, afloat, and that encounter the open boundary in backward and forward time are reported.
The connection percentage, or percentage of particles reaching a coastline segment from a given transect, provides indications of coastal areas that could serve as sources and sinks.
The residence time is defined as the total duration of a particle in the Adriatic marine environment and is the sum of the time afloat in backwards and forwards time. Particles that remained afloat at the end of the± 60 day integration period were not included in estimates of the residence time. Bootstrap estimates of the averages (Efron and Tibshirani, 1986) are used as the distributions of residence times are non-normal.
FIGURE 5 | Selected backwards particle trajectories based on observations of floating anthropogenic marine debris on 3 March 2015 (A),4 March 2015 (B), 9 March 2015(C), and 11 March 2015(D). The release locations are indicated by red circles and the color scale corresponds to time in days since initial release.
3.2. Debris Tracking
3.2.1. May 2013: Central and Southern Adriatic Sea Sixty two, twenty five, and thirteen percent of backward particle beached, remained afloat, or encountered the open boundary, respectively (Table 2). The average time from source to sighting location was 22.8 days. The forward trajectories suggest significant export, with 46% of particles encountering the open boundary. 41% of particles beached and 13% were still afloat after 60 days. The average time from sighting location to a coastal sink was approximately 39 days.
Example backward trajectories reveal spatial and temporal variability in surface transport pathways. AMD observed 9 May 2013 at approximately 42◦N, 16◦E (Figure 2A) were transported by the EAC, the central cyclonic sub-gyre, and the WAC while debris observed 9 days later and about 100 km to the east (42◦N, 17◦ E) originated almost exclusively from the southern cyclonic sub-gyre (Figure 2C). AMD observed near the east coast
originated from the nearby coastline and were transported by the EAC, with a small subset from the southern sub-gyre (Figure 2B).
Backwards trajectories suggest that AMD observed 19 May at approximately 15◦N, 43.5◦E originated from the east and west coasts and were transported in the cyclonic central sub-gyre.
Example forward trajectories show the influence of the same large-scale features, namely the EAC, WAC, and the southern gyre (Figures 3A–C). However, AMD observed 19 May at approximately 15◦N, 43.5◦E traveled north toward the Croatian coastline before looping west until they were entrained in the WAC and transported alongshore toward the Gargano Peninsula (Figure 3D).
The backwards and forwards connection percentages show that a continuous 400 km stretch of the central Italian coast (segments 7–14) acted as both sources and sinks (Figure 4).
Segment 4, near Bari, provided more than 80% of the AMD sighted on transect 11. Much of the Croatian shoreline acted as
FIGURE 6 | Selected forward particle trajectories based on observations of floating anthropogenic marine debris on 3 March 2015 (A),4 March 2015 (B), 9 March 2015(C), and 11 March 2015(D). The release locations are indicated by red circles and the color scale corresponds to time in days since initial release.
both source and sink as well. Import and export of observed AMD across the open boundary is also likely, with large connection percentages observed along the open boundary segment for both backward and forward particle trajectories.
Winds during the backwards trajectories (9 March–9 May) were highly variable with several strong, sustained Sirocco events interspersed with weaker Mistral winds (Figure 4D).
Winds remained variable, with a weaker Sirocco event, during visual debris sightings before transitioning to weaker, but more persistent Mistral winds from June to August 2013 (Figure 4D).
Particles afloat after −60 days were distributed between the central and southern cyclonic sub-gyres while the 13% of particles afloat after+60 days were concentrated largely in the southern gyre (Figures 11A,B).
3.2.2. March 2015: Southern Adriatic Sea
Forty one, seventeen, and forty two percent of backwards particles beached, remained afloat, and originated from the
open boundary, respectively, within −60 days of observation (Table 2). The average time from source to sighting location was 25 days. Thirty five, forty, and twenty five percent of forward particles beached, remained afloat, and reached the open boundary, respectively, within +60 days of observation.
The average time from sighting location to coastal sink was 31.4 days. Example backwards trajectories reveal the influence of the southern cyclonic gyre and, to a lesser extent, the WAC and EAC (Figure 5). Forward trajectories suggest export via the WAC (Figures 6A,C) and recirculation in the southern gyre accompanied by limited exchange with the central Adriatic via the EAC (Figures 6B,D).
Connection percentages (Figure 7) show that a 200 km stretch of the Italian coastline from the Otranto Strait to south of the Gargano Peninsula (segments 1–5) acted as both sources and sinks for the observed debris. The central Italian coastline, from the Gargano Peninsula to Conero (segments 7–13) acted as sources. Much of the central, eastern coast, from 42◦N to 44◦N
FIGURE 7 | Percentages of particles from a given transect (rows) that reach a given coastal segment (columns) for backward (A)and forward(B) trajectories computed from the March 2015 debris.(C)Coastline segments that served as both sources and sinks are plotted in dark blue. Light blue segments acted as sources only and orange segments acted as sinks. Transect locations and numbers are shown in red.(D)Stick plot of winds during the particle tracking experiment. Red (blue) indicates Sirocco (Mistral) winds and the vertical dashed lines denote the time period of the AMD observations.
acted as sinks. The southeastern coast acted as sources and sinks, with segment 33 on the Albanian coast providing over 80% of debris sighted on transects 13 and 14. Winds throughout the entire period were highly variable, with alternating Mistral and Sirocco events (Figure 7D).
3.2.3. November 2015: Northern Adriatic Sea
Backwards particle trajectories suggest that sighted debris originated within the Adriatic, as none of the particles came from the open boundary (Table 2, Figures 8, 10A). Eighty eight and twelve percent of backwards particles were beached and still afloat, respectively, within −60 days of sighting (Table 2). The average time from source to sighting location was approximately 23 days. Example backward trajectories illustrate the complex, unpredictable nature of Lagrangian transport (Figure 8). Particles were transported by the EAC and the northern arm of the central cyclonic gyre, as well as the southern gyre (Figure 8). Backwards connection percentages confirm that most of the sighted debris originated from the central and northern Adriatic (Figure 10A). On the central Italian coastline, the most active source regions were segments 10 and 14 (Figure 10A). In the northern Adriatic, the Istrian Peninsula was also an active source (segments 21–24). Segment 38 was the most active source on the east coast, corresponding to the Croatian Island of Dugi otok.
Forward trajectories also remained largely in the Adriatic region, with 82% beached and 17.9% afloat within +60 days
of sighting (Table 2). A very small subset (0.1%) of forward particles reached the open boundary. The average time from coastal sink to sighting location was 21.5 days. Example forward trajectories show unexpected northward transport (Figure 9A) and recirculation (Figure 9B), as well as more typical alongshore transport in the WAC (Figures 9C,D). Connection percentages show that the central and southern Italian coastline acted as sinks, with the Po River delta (segment 18) and the northern Gargano Peninsula (segments 7–8) receiving much of the sighted debris (Figure 10B). Strong Sirocco wind events were likely responsible for deviations from mean circulation patterns. For example, the reversal of particles observed in forward particles (Figure 9A) was likely driven by upwelling-favorable Sirocco winds (Figure 10D).
3.3. General Summary
Overall, the observed AMD originated largely from coastal segments near population centers and major rivers and was transported by the cyclonic surface circulation until either stranding, exiting the Adriatic, or recirculating in the southern gyre (Figures 2–9). Overall, 63, 16, and 21% of backwards particles beached, remained afloat, or originated from the open boundary, respectively (Table 2). In forward time, 55, 27, and 18% of particles beached, remained afloat, or were transported to the open boundary. The average residence time was 47.2 days and was slightly longer in forward time (24.3 days) when compared to backward time (22.9 days).
FIGURE 8 | Selected backwards particle trajectories based on observations of floating anthropogenic marine debris on 20 November 2015 (A),18 November 2015(B), 20 November 2015(C), and 20 November 2015(D). The release locations are indicated by red circles and the color scale corresponds to time in days since initial release.
Of the 64 coastal segments used to identify possible sources and sinks of floating AMD in the Adriatic Sea, 41 segments acted as sources and 32 acted as sinks (Figure 1). The Venice lagoon segment did not serve as a source or sink for the observed floating AMD in any of the three experiments. The remaining unaffected coastal segments were concentrated in the northeastern Adriatic on the inshore, eastern sides of islands and peninsulas (Figure 1).
The results suggest that the central and southern gyres could have supplied AMD in May 2013 and March 2015 (Figure 11).
Similarly, particles remained in the southern gyre in all three experiments after +60 days, suggesting that these regions can act as retention zones. However, the finite and relatively short integration period does not accurately reflect the lifetimes of AMD, especially plastics, and this restriction is addressed in Section 4.2.